pschwenn
New member
In an earlier topic, three aspects of Wings (Winglets on Keels and Wings) were missed or understated, which might simplify understanding some of the questions raised. Span constraints, inherent disadvantage, and design subtlety and difficulty.
Span Contraints. Winglets on airliners might seem to be powerful evidence of their performance enhancement. No; initially airports did not have enough large enough docking spaces for a wave of new aircraft on the way. In boating, along with the multiplicity of old-age constraints on draft, the 12-metre AC rule had a 9' draft limitation. The winglets on the AC boats fit the boats to the rule, deeper keels would have been superior (for performance) in all respects. The winglets on airliners, initially, fit them into their parking spaces.
The 787, freed by larger spaces, exploits the advantage of not having winglets. Its outer span has subtle shaping and shape changing flexibility which add to that advantage in a way that tacking on even clever winglets to unconstrained span do not - the A380 has little winglets both for marketing and to simplify adding virtual span long after the initial design. Winglets are being marketed as well to all sorts of smaller General Aviation aircraft - both for span constraints in existing hangars, and for marketing, and, again, and ultralight tack-on is simpler in every way than redesign and renovation of an existing wing.
Difficulty & Subtlety of Increasing Effective Span. Draft constraints on keels can be sharp, or just desirable, so the search for lower CG and greater effective span is quite legitimate. But very difficult to get right. Scheel headed in the right direction, but understandably, missed. If someone other than Burt Rutan designs a Canard airplane, watch out. If a team tacks winglets on a 12 metre without the insight and analysis of the early Australian boat, hold that bet. The twin keels are a great alternative either side of the English Channel for sitting down gracefully when the tides out without resorting to stilts, but they are not a match for performance for the same effective deeper single span (and sitting down can be a lot worse than graceful.)
If Scheel had gone further: a gentler, truly progressive increase in foil thickness toward the base of the keel, and a near-sharp edge at the base as virtual fence (with zero surface area) for flow (pressure) escaping below, before reaching the trailing edge. Perhaps progressing to a foil at the base which is net more effective at higher foil thickness. It might have worked quite well. Searching for lower VCG, "Marry it or Divorce it": a keel should lift, a bulb should be as invisible as possible (not easy), not lifting or dragging more than inevitable.
Inherent Disadvantage. Winglets will only get you a virtual span increase of 1/2 or a little more of their own span. And they'll generally have twice the surface area (or more), of simple span increase. They'll introduce interference drags the simple span lacks.
Advances in other keel shaping to reduce induced drag (the "bad" wake vortices) might change those deficits. You won't have to wait for an example: Centerboards.
At first sight, a long post wouldn't seem to simplify anything, but sometimes seeing that a situation is simpler than it appears gets complicated.
regards,
Span Contraints. Winglets on airliners might seem to be powerful evidence of their performance enhancement. No; initially airports did not have enough large enough docking spaces for a wave of new aircraft on the way. In boating, along with the multiplicity of old-age constraints on draft, the 12-metre AC rule had a 9' draft limitation. The winglets on the AC boats fit the boats to the rule, deeper keels would have been superior (for performance) in all respects. The winglets on airliners, initially, fit them into their parking spaces.
The 787, freed by larger spaces, exploits the advantage of not having winglets. Its outer span has subtle shaping and shape changing flexibility which add to that advantage in a way that tacking on even clever winglets to unconstrained span do not - the A380 has little winglets both for marketing and to simplify adding virtual span long after the initial design. Winglets are being marketed as well to all sorts of smaller General Aviation aircraft - both for span constraints in existing hangars, and for marketing, and, again, and ultralight tack-on is simpler in every way than redesign and renovation of an existing wing.
Difficulty & Subtlety of Increasing Effective Span. Draft constraints on keels can be sharp, or just desirable, so the search for lower CG and greater effective span is quite legitimate. But very difficult to get right. Scheel headed in the right direction, but understandably, missed. If someone other than Burt Rutan designs a Canard airplane, watch out. If a team tacks winglets on a 12 metre without the insight and analysis of the early Australian boat, hold that bet. The twin keels are a great alternative either side of the English Channel for sitting down gracefully when the tides out without resorting to stilts, but they are not a match for performance for the same effective deeper single span (and sitting down can be a lot worse than graceful.)
If Scheel had gone further: a gentler, truly progressive increase in foil thickness toward the base of the keel, and a near-sharp edge at the base as virtual fence (with zero surface area) for flow (pressure) escaping below, before reaching the trailing edge. Perhaps progressing to a foil at the base which is net more effective at higher foil thickness. It might have worked quite well. Searching for lower VCG, "Marry it or Divorce it": a keel should lift, a bulb should be as invisible as possible (not easy), not lifting or dragging more than inevitable.
Inherent Disadvantage. Winglets will only get you a virtual span increase of 1/2 or a little more of their own span. And they'll generally have twice the surface area (or more), of simple span increase. They'll introduce interference drags the simple span lacks.
Advances in other keel shaping to reduce induced drag (the "bad" wake vortices) might change those deficits. You won't have to wait for an example: Centerboards.
At first sight, a long post wouldn't seem to simplify anything, but sometimes seeing that a situation is simpler than it appears gets complicated.
regards,
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